Full Blood Count (FBC)

Question 1

What is the aim of an FBC?

Answer 1

To evaluate the composition and concentration of the cellular components of blood

Question 2

List some indications for ordering an FBC?

Answer 2

Indications:
• Pre-operative: ensuring adequate O2 carrying capacity and haemostasis
• Diagnose infection and WBC disorders (e.g. leukaemia)
• Diagnose anaemia, monitor anaemia treatment
• Diagnose bleeding disorders
• Diagnose acute and chronic illness
• Monitor effects of chemo and radiotherapy on haemapoiesis

Question 3

Describe the process of specimen collection for an FBC?

Answer 3

• Venous blood sample collected in an EDTA tube (Ethylenediaminetetraacetic acid, strong anticoagulant)
• EDTA chelates calcium -> preventing coagulation

Question 4

Describe aseptic venepuncture technique?

Answer 4

Venepuncture:

• consent
• safety check (medical conditions, bleeding diatheses)
• gather materials, check expiry
• wash hands, gloves
• prepare site: tourniquet and find site (cephalic, basilica, median cubital v- palpable and fixed), release tourniquet and prepare materials, reapply tourniquet to venepuncture
• clean with alcohol, dry
• collet specimen: fix vein, approach along vein, bevel up, 1 degrees, watch for flash back, fill all tubes completely
• post-procedure: pressure on site for 3mins, inspect regularly label samples
• specimen storage if testing delayed, room temp for 8hrs

Question 5

Describe the specimen analysis of a FBC?

Answer 5

Specimen barcoded -> placed in automated analyser
⇒ Sample (150micoL) aspirated into chamber, diluted w balanced isotonic saline solution
⇒ Split into 3 different chambers: Erythrocytes and platelets, Haemoglobin (reagent lyses RBCs to emasure Hb released), Leukocytes
⇒ Small amounts of each fluid passes through small aperture
⇒ Current applied by 2 electrodes -> cells act as resistors and increase the impedance/effective resistance according to size -> pulse created with amplitude proportional to size (Coulter Principle)
⇒ Different cells produce different impedances -> machine groups them according to thresholds (RBC >36fL, platelet 2-20fL) -> identifying cell types
⇒ RBC data: forms bell curve
- Area under the curve is the RCC
- Width is RDW
- Mean is the MCV
⇒ WBCs: clusters on a 3D histogram correspond to the different types of leukocytes

Question 6

How are internal structural features of RBCs analysed?

Answer 6

• Automated blood cell counters measure RBC number, MCV and Hb concentration
⇒ Hc, MCH (mean cell Hc) and MCHC (mean cell Hc conc) are derived from these primary measures

• Flow cytometry: incident laser beam strikes passing cells (each with unique ability to absorb and refract light depending on cytoplasmic and nuclei constituents) -> light scatters at different angles
⇒ Multivariate analysis of cytometry data informs cell count, volume and internal structure

Question 7

List some limitations of a FBC?

Answer 7

FBC limitations:
• Abnormal cells may be incorrectly identified
• RBC may not lyse properly
• Large platelets may be misidentified
• Laser unable to detect scatter in lipaemic (high lipid conc) samples
• Confounded by drugs increasing/decreasing RBC, WBC and/or platelet production (consider medications in result interpretation)